Method for making particulate compositions and products thereof

ABSTRACT

A method of making abrasive compositions, and more particularly, it relates to a method of making precipitated silica abrasive compositions having excellent cleaning performance and lower abrasiveness with post-reactor sizing of the abrasive particles being performed directly via wet comminution and centrifugation, optionally followed by hydraulic chamber press filtering combined with vacuum dewatering and de-agglomeration is provided. By targeting a specific particle size range, it has been determined that higher pellicle film cleaning levels may be achieved without also increasing the dentin abrasion properties of the silica products themselves. As a result, dentifrices including such classified abrasive silica products, exhibiting particularly desirable cleaning benefits, can be provided for improved tooth polishing, whitening, and the like, without deleteriously affecting the hard tooth surfaces. Also encompassed within this invention also are products of this selective process scheme and dentifrices containing such particularly manufactured and classified silica products.

FIELD OF THE INVENTION

This invention relates to a method of making abrasive compositions, andmore particularly, it relates to a method of making precipitated silicaabrasive compositions having excellent cleaning performance and lowerabrasiveness with post-reactor sizing of the abrasive particles beingperformed directly via wet comminution and centrifugation, optionallyfollowed by hydraulic chamber press filtering combined with vacuumdewatering and de-agglomeration. By targeting a specific particle sizerange, it has been determined that higher pellicle film cleaning levelsmay be achieved without also increasing the dentin abrasion propertiesof the silica products themselves. As a result, dentifrices includingsuch classified abrasive silica products, exhibiting particularlydesirable cleaning benefits, can be provided for improved toothpolishing, whitening, and the like, without deleteriously affecting thehard tooth surfaces. Also encompassed within this invention also areproducts of this selective process scheme and dentifrices containingsuch particularly manufactured and classified silica products.

BACKGROUND OF THE INVENTION

Toothpaste manufacturers strive to produce dentifrices with highcleaning and low abrasivity. Such formulators achieve this goal byincorporating abrasive substances into the toothpaste formulation. Anabrasive substance has been included in conventional dentifricecompositions in order to remove various deposits, including pelliclefilm, from the surface of teeth. Pellicle film is tightly adherent andoften contains brown or yellow pigments, which impart an unsightlyappearance to the teeth. While cleaning is important, the abrasiveshould not be so aggressive so as to damage the teeth. Ideally, aneffective dentifrice abrasive material maximizes pellicle film removalwhile causing minimal abrasion and damage to the hard tooth surfaces.Consequently, among other things, the performance of the dentifrice ishighly sensitive to the abrasive polishing agent ingredient.

A number of water insoluble, abrasive polishing agents have been used ordescribed for dentifrice compositions. These abrasive polishing agentsinclude natural and synthetic abrasive particulate materials. Thegenerally known synthetic abrasive polishing agents include amorphousprecipitated silicas, silica gels, dicalcium phosphate and its dihydrateforms, calcium pyrophosphate and precipitated calcium carbonate (PCC).Other abrasive polishing agents for dentifrices have included chalk,magnesium carbonate, zirconium silicate, potassium metaphosphate,magnesium orthophosphate, tricalcium phosphate, and the like.

Synthetically produced amorphous precipitated silicas, in particular,have been used as abrasive components in dentifrice formulations due totheir cleaning ability, relative safety, and compatibility with typicaldentifrice ingredients, such as humectants, thickening agents, flavoringagents, anti-caries agents, and so forth. Synthetic precipitated silicasgenerally are produced by the de-stabilization and precipitation ofamorphous silica from soluble alkaline silicate by the addition of amineral acid and/or acid gases under conditions in which primaryparticles initially formed tend to associate with each other to form aplurality of aggregates (i.e., discrete clusters of primary particles),but without agglomeration into a three-dimensional gel structure. Theresulting precipitate is separated from the aqueous fraction of thereaction mixture by filtering, washing, and drying procedures, and thenthe dried product is mechanically comminuted in order to provide asuitable particle size.

Such previously produced and utilized precipitated silica abrasives havebeen produced and provided for dentifrices generally in terms of overallcleaning and abrasive qualities. Although such previous products haveaccorded excellent benefits in these areas, it has been noted thatcertain limitations in terms of production costs and waste generationare prevalent as a result. For instance, in order to target specificlower abrasive levels, milled particles include materials that exhibitparticle sizes outside the required ranges. Proper filtering anddisposal of such undesirable materials are thus needed. The same typesof production issues and problems arise when targeting certain lowerabrasive levels for particulate materials without sacrificing pelliclefilm cleaning as well. As it concerns the ability to provide less costlyproduction methods for providing such effective low-abrasion, highpellicle film cleaning materials to users susceptible to unwanted dentinabrasion at the gum line, as well as potential supplementalabrasive/cleaning silica products for more effective polishing and/ortooth whitening applications, the industry has been reliant uponoutdated methods of production, separation, and disposal of undesirableparticles. As a result, there are areas within the dental silicamaterials industry in which improvements to such ends are desired.

Given the foregoing, there is a continuing need for a method ofproducing precipitated silica materials that provide excellent cleaningperformance, but with lower abrasivity values, that can be included in atoothpaste composition. To that end, the following invention has provento accord such coveted results.

BRIEF SUMMARY OF THE INVENTION

It is thus one advantage of this invention to provide an all-inclusivemethod of manufacturing dental abrasive particles that exhibit properparticle size ranges for effective pellicle film cleaning and moderatedentin abrasion characteristics once manufacturing is completed and saidparticles are thus collected. Thus, no dry post-reactor comminution orother type of particle modification is necessary to provide the desiredparticle size ranges for effective high cleaning and moderate dentinabrasion properties exhibited by the produced dental abrasives.

The above and other advantages and benefits are achieved by the presentinvention directed to a method of making silica compositions with watercontent reduction and silica particle comminution effected duringpost-reactor processing under wet conditions.

The basic inventive method entails the production of abrasivecompositions, comprised of water-insoluble abrasive polishing agentssuspended in an aqueous medium, wherein milling to desired particle sizeranges is effectuated via a wet process during actual abrasive productmanufacture. Such a method thus avoids the need and associated cost ofdry milling the abrasive particle content, and products thereof. It hasalso been realized that such a wet milling method permits production ofappropriately sized abrasive particles and compositions thereof that arerheologically stable, are not prone to settling, and do not exhibit anyappreciable level of re-agglomeration, even during and after transportand/or storage before end-use, such as incorporation into dentifriceformulations or other oral cleaning compositions.

Although any known particulate abrasive for dentifrices may be utilizedwithin this invention, particularly preferred are amorphous precipitatedsilica abrasives. Abrasive compositions including such individualparticles should comprise a plurality of silica particles exhibiting amedian particle size of about 5 to about 15 microns, preferably fromabout 6 to about 10, and more preferably from about 7 to about 9, aparticle size span of less than or equal to 2, preferably from about1.25 to about 2.0, and more preferably from about 1.25 to about 1.95.

The invention also includes a dentifrice comprising about 5 wt % toabout 35 wt % of the amorphous precipitated silica composition notedabove and produced in accordance with such an inventive method, andexhibiting an radioactive dentin abrasion (RDA) level between about 130and 200 (preferably from about 130 to about 195) and a pellicle filmcleaning ratio (PCR) of between about 70 and 140 (preferably from about80 to about 140).

Basically, it has been realized that the inventive method provides thecapability of easily producing low-structure abrasive silica materialswithin a concentrated range of specific particle sizes permits greateruniformity in performance during tooth cleaning with a dentifricecontaining such materials with minimal process steps and thusconcomitant lower associated costs. Likewise, providing such materialswithin the specific range of particle sizes permits targeting particularareas of tooth surfaces for proper cleaning without simultaneouslyexhibiting excessive abrasive levels.

DETAILED DESCRIPTION OF THE INVENTION

All parts, percentages and ratios used herein are expressed by weightunless otherwise specified. All documents cited herein are incorporatedby reference. The following describes preferred embodiments of thepresent invention, which provides silica for use in dentifrices, such astoothpastes. While the optimal use for this silica is in dentifrices,this silica may also be used in a variety of other consumer andindustrial products, such as cosmetics and coatings. In these otherproducts, the same process is used, however a different particle sizefraction of particles is isolated, such as smaller particles, dependingon the final use requirements.

By “mixture” it is meant any combination of two or more substances, inthe form of, for example without intending to be limiting, aheterogeneous mixture, a suspension, a solution, a sol, a gel, adispersion, or an emulsion.

By “dentifrices” it is meant oral care products such as, withoutintending to be limiting, toothpastes, tooth powders and denture creams.

For the purposes of defining this invention, the term “particle sizespan” is intended to mean the cumulative diameter of the particles inthe tenth volume percentile (D10) minus the cumulative volume at theninetieth percentile (D90) divided by the diameter of the particles inthe fiftieth volume percentile (D50), i.e. (D10-D90)/D50. A lower spanvalue indicates a narrower particle size distribution.

The present invention relates, particularly, to amorphous, precipitatedsilica compositions, also known as silicon dioxide, or SiO₂, whichimpart improved cleaning and abrasive characteristics when includedwithin a toothpaste or dentifrice. These abrasive silicas not only cleanteeth by removing debris and residual stains, but also function topolish tooth surfaces. Because the silicas of the present invention havebeen classified to remove fine particles which are believed to have lesscleaning benefit and large particles which are believed to contribute toincreased abrasion, they have a narrower particle size distribution andare particularly useful for formulating a dentifrice that has excellentcleaning with lower abrasivity. Other possible abrasive products thatmay be produced through the inventive process include, withoutlimitation, precipitated calcium carbonate (PCC), silica gels, dicalciumphosphate and its dihydrate forms, and calcium pyrophosphate. Suchparticles are engineered to provide excellent cleaning benefits fordental applications. Such cleaning characteristics are generallymeasured as pellicle film cleaning ratios (PCR), and are discussed ingreater detail below, particularly in conjunction with other abrasivemeasurements.

A sufficient amount of abrasive silica should be added to a toothpastecomposition so that the radioactive dentin abrasion (“RDA”) value of thetoothpaste is between about 50 and about 250. At a RDA of less than 50,the cleaning benefits of the toothpaste will be minimal, while at a RDAof greater than 250, there is risk that the toothpaste will be soabrasive that it may damage the tooth dentin along the gum line.Preferably, the dentifrice should have a RDA value of at least about 50,such as between about 70 and 200.

The RDA of a toothpaste is dependent on the hardness of the abrasive,the abrasive particle size and the concentration of the abrasive in thetoothpaste. The RDA is measured by the method described in the article“A Laboratory Method for Assessment of Dentifrice Abrasivity”, John J.Hefferren, in Journal of Dental Research, Vol. 55, no. 4 (1976), pp.563-573. Silica abrasivity or hardness can also be measured by anEinlehner method, which is described in greater detail below.

By the present invention, abrasive amorphous silicas have been developedthat not only have excellent cleaning performance (in terms of PCRmeasurements) but are also less abrasive than typically producedproducts. By using the inventive wet milling, all-inclusive abrasiveparticle composition production method, an abrasive material (such as,preferably, though not necessarily an amorphous precipitated silicaproduct) may be produced that has relatively low RDA and Einlehnerabrasion values over a given PCR range.

Accordingly, the particulate compositions (and the preferred abrasiveparticles, and more preferred abrasive amorphous precipitated silicacompositions) of the present invention are prepared according to thefollowing first process comprising the sequential steps of:

a) providing a plurality of particles selected from the group consistingof precipitated silica particles, silica gel particles, precipitatedcalcium carbonate particles, calcium pyrophosphate particles, dicalciumphosphate, and any mixtures thereof;

b) subjecting said plurality of particles to a comminuting step in a wetenvironment;

c) subjecting said wet-comminuted particles to a particle sizeclassification step wherein particles exhibiting a median particle sizerange of from about 5 to about 30 microns are collected;

d) subjecting said collected particles from step “c” to a subsequentparticle size classification step wherein particles exhibiting a medianparticle size range of from about 5 to about 15 microns are collected;optionally

e) subjecting said collected classified particles from step “d” to adewatering step wherein said dewatered particles exhibit a moisturecontent of at most 60 wt %; and, optionally

f) subjecting said dewatered abrasive particles to a deagglomerationstep. Such process steps are discussed in greater detail below.

Alternatively, a second inventive method may encompass the sequentialsteps of:

a) providing a plurality of precipitated silica particles that have notpreviously been dried prior to commencement of step “b”;

b) subjecting said plurality of particles to a comminuting step in a wetenvironment;

c) subjecting said wet-comminuted particles to a particle sizeclassification step wherein particles exhibiting a median particle sizerange of from about 0.1 to about 15 microns are collected;

d) subjecting said collected classified particles from step “c” to adewatering step wherein said dewatered particles exhibit a moisturecontent of at most 60 wt %; and, optionally

e) subjecting said dewatered abrasive particles to a deagglomerationstep.

Such process steps are also discussed in greater detail below.

In the first step of the first method outlined above (and as can befollowed for the second method), at least for the production ofprecipitated silica abrasives, an acidulation reaction is performed toprecipitate silica. The initial acidulation reaction is performed in areaction system equipped with suitable heating equipment. In general,the precipitated silicas made in step “a” may be prepared by a freshwater, or electrolyte solution, acidulation process wherein silica isprecipitated by reaction of an alkali metal silicate and a mineral acidin aqueous solution. In the fresh water process, no electrolyte such asalum, Na₂SO₄, or NaCl, is present during the acidulation reaction.

A portion of the sodium silicate solution is charged to a reactorcontainer or chamber including agitator means to provide agitation tothe container contents. Preferably, about 0% to 30% of the totalstoichiometric amount of sodium silicate solution is placed in thereactor container to serve as initiating nuclei for the silica. Theaqueous solution of sodium silicate in the container is then preheatedto a temperature in the range of about 60 to 100° C., more preferablyabout 80 to 95° C. Prior to introduction into the reactor container, theremaining sodium silicate is preferably preheated to about 70 to 95° C.The acid solution temperature is preferably ambient.

Although sodium silicate is illustrated, it will be understood that anysuitable alkali metal silicate could be used. The term “alkali metalsilicate” includes all the conventional forms of alkali silicates, asfor example, metal silicates, disilicates and the like. Water solublepotassium silicates and sodium silicates are particularly advantageouswith the latter being preferred. It should be taken into considerationthat the mole ratio of the alkali silicate, i.e., the ratio of silica toalkali metal oxide, contributes, depending on other reaction parameters,to the average pore size of the silica products. In general, acceptablesilica products of this invention can be made with silicate molar ratios(SiO₂:Na₂O) ranging from about 1.0 to 3.5 and preferably from about 2.4to about 3.4. The alkali silicate solution supplied to the reactorvessel during various processing steps in the inventive method, asdescribed elsewhere herein, generally can contain between about 8 to35%, and more preferably between about 8.0% and 25.0%, by weight alkalimetal silicate based on the total weight of the alkali metal silicatesolution. In order to reduce the alkali silicate concentration of asource solution of alkali silicate to the above-indicated desired range,dilution water can be added to a source solution of alkali silicatebefore the silicate solution is fed into the reactor, or, alternatively,the dilution water can be combined in situ with the source solution ofalkali silicate in the reactor used in the acidulation reaction step “a”with agitation-mixing to formulate the desired concentration of silicatein the alkali metal silicate solution.

The acid, or acidulating agent, can be a Lewis acid or Brönsted acid,and preferably is a strong mineral acid such as sulfuric acid,hydrochloric acid, nitric acid, phosphoric acid, and so forth, and morepreferably sulfuric acid, added as a dilute solution thereof (e.g., at aconcentration of between about 6 to 35 wt %, more typically about 9.0 to20.0 wt %).

Once the reactor solution and remaining reactants have reached thedesired temperatures, simultaneous addition of the remaining sodiumsilicate solution and acid into the reactor is commenced. The sodiumsilicate solution and acid are metered into the reactor over an additiontime of about 30 to 90 minutes. Rates of addition of the reactantsdepend upon the mole ratio, addition time and concentration of thesilicate and the concentration of the acid.

At the end of this co-addition period, most of the silica hasprecipitated and the sodium silicate addition is stopped. Addition ofthe acid is continued until the reactor slurry reaches the desired pH.Once the slurry pH reaches about 7.0, it is preferable to reduce theacid flow rate until the slurry pH approaches the target pH, at whichpoint the acid flow can be stopped and manual adjustment used to reachthe target slurry pH. The preferred slurry pH is approximately 4.0 to7.0, and more preferably between 5.0 to 6.0. At this juncture, thesilica has precipitated to provide a mixture of the precipitated silicaand the reaction liquor. Once the desired slurry pH is reached,digestion begins and the reaction temperature is raised to approximately85-99° C., and preferably 91 to 97° C., and digestion is continued atthe elevated temperature for approximately 5 to 60 minutes, andpreferably for approximately 10 to 20 minutes. Acid is added during thedigestion step to the extent necessary to maintain a constant pH.

After the digestion step is completed in the reactor, and any subsequentpH adjustment conducted, the reaction batch is discharged from thereactor. Although the above-described general protocol are preferred forsynthesizing the precipitated silica to be conditioned according to thisinvention, it will be appreciated that other grades of precipitatedsilicas, such as very low to high structure synthetic silicas inaccordance with the definitions set forth in J. Soc. Cosmet. Chem., 29,497-521 (August 1978), and Pigment Handbook: Volume 1, Properties andEconomics, 2nd ed., John Wiley & Sons, 1988, p. 139-159, generally canbe used in the practice of this invention.

In the second method, at least, it is imperative that the subject silicaparticles not be subjected to any drying prior to step “b” commencing.Basically, if the particles have been dried previously, the structureprior to comminution (wet or otherwise) would be significantly differentfrom that needed for the inventive method to ultimately provide suitableprecipitated silica particles for the noted end uses. Such a limitation,though not necessary within the first method discussed, is nonetheless apotentially preferred requirement therein as well.

In step “b” of either the first or second method, wet comminuting of thereaction mass is performed. Comminution is needed because the silicaparticles suspension drawn from the reactor of step “a” generally have amedian particle size (MPS) of greater than about 50 μm to about 100 μm,and more typically about 65 μm to about 85 μm. These particles sizes areunacceptable for applications such as cosmetics, coatings and oralcleaning compositions. Namely, smaller silica particles are needed sothat the particles are not gritty in texture to a user, yet for oralcare use the particles must be large enough to provide the requisitepolishing action on teeth. For oral cleaning compositions, silicaparticle sizes between about 1 and about 30 μm are generally required,and a median particle size of between about 3 to 15 μm is preferred inthis invention. For cosmetic and coating uses, the median particle sizerange would be smaller, such as from 0.1 to about 3 microns. Thus, arange of from 0.1 to 15 is suitable for the products of this invention(more particularly between 0.3 and 10; for preferred oral care end-uses,the range is preferably from 7 to 9 microns).

Prior to the wet comminution (i.e., wet milling) step of either first orsecond method, it is important that the silica particles not besubjected to any prior controlled milling procedures (such controlledmilling would not include shearing during particle production within areactor). The controlled comminution of the silica particle in a wetenvironment permits greater reliability in producing the desiredparticle size ranges as well as reliability in terms of modifying eachparticle in the same manner and under the same conditions.

Such comminution is performed, as noted above, through a wet grindingstep in a separate station from particle production, or, as analternative, via high shear reactor mixing during and/or after silicaparticle production. Preferably, in order to comminute the abrasiveparticles (typically agglomerates) in step “b”, the particles(preferably, though not necessarily, precipitated silica particles) ofstep “a” are fed to a wet media grinding station. Either a single stagewet media mill or a multi-stage wet milling operation in step “b” can beused. For example, the multi-stage wet media grinding station, in oneembodiment, can be comprised of two or more separate mills through whichthe slurry is successively progressed. Alternatively, the multi-stagewet media grinding station can be comprised of a single mill in whichthe slurry is fed through the single mill in multiple passes usingrecirculation. The amount of energy dissipated into the feed slurry ateach mill stage, or in each pass through a single mill in a multi-passform of multi-stage milling, generally is kept approximately the same,although this is not necessarily required. Multi-stage wet media millingpermits longer residence times to be applied.

The wet media mill types used as the mill or mills described above inthe multi-stage grinding station independently can be ball mills, wetvertical media mills, wet horizontal media mills and the like. Onepreferred type of wet grinding mill used in the practice of thisinvention is a Model HML 1.5 Premiere Mill manufactured by Lightnin,Inc., Reading, Pa. The Premier mill is a horizontal style media mill.The milling media used preferably are ceramic beads, e.g., zirconiumoxide beads, of about 1 to 3 mm in size, which are loaded in the millsat about 20 to 80 vol %.

In one preferred non-limiting illustration, the wet bead mill used toconduct step “b” generally is operated under the following conditions:

Bead loading: 20-60%; and

Bead mill rotor speed: 500-3500 FPM (feet per minute) (152-1067m/minute).

In keeping with an objective of this invention of reducing silicaparticles without the need for drying and dry milling procedures, thetotal amount of shearing forces applied to the slurry or fluidized presscake during wet grinding should be sufficient to reduce the medianparticle size (MPS) to between about 0.1 to about 30 microns (μm),preferably between about 1 and about 25 microns, and more preferablybetween about 3 and about 15 microns. The abrasive particles in the wetmilled abrasive composition have less than 1.5 wt % fraction ofparticles greater than 45 μm (+325 mesh). Of course, the millingconditions can be adjusted to achieve the particle size desired for aparticular application.

Alternately, the silica particles of step “a” are first filtered andwashed before being fed to the grinding station of step “b” (if such agrinding station is used). The silica reaction slurry produced in step“a” typically contains about 6 to 12 wt. % silica particles, which maybe increased to about 12 to 50 wt. % silica particles by filtration. Theparticles may be collected and washed to remove reaction by-products(e.g. sodium sulfate) on any batch or continuous filtration device knownto one skilled in the art, such as vacuum and pressure filters, e.g.rotary filter, belt filter, Larox filter, plate and frame filter, orfilter press.

In step “c” or either of the first or second methods outlined above, thecomminuted particles (preferably, milled precipitated silica particles)of step “b” were thereafter fed at a rate of about 0.5-10 LPM into adecanter centrifuge, such as a 6-inch solid bowl continuous flow Birddecanter/centrifuge available from Bird Machine Company, Inc., SouthWalpole, Mass., to isolate particle fractions of desired size. Thedecanter centrifuge, operated at 20-70 HZ, can be configured with bothof the underflow (larger particles) and overflow (smaller particles)streams recirculated or only one flow recirculated to thecentrifuge/decanter or to the mill. The milling-centrifuge/decantercombination may be either a single stage combination or a multi-stagecombination. The inventive process contemplates isolating smallerparticle fractions, larger particle size fractions and isolatingparticles that have been “double cut”, that is, where particles above adesired particle size range are removed and particles below a desiredparticle size range are removed leaving only particles of a narrowdesired particle size range.

In one embodiment, the larger particles from the centrifuge/decanterunderflow stream is fed into a mill for further particle size reductionand thereafter fed back into a centrifuge decanter. This “closed loop”system has the effect of continuously reducing the particle size of thelarger undesirable particles and the recycling continues until desiredparticle size ranges are obtained. The time necessary to achieve desiredparticle size ranges depends upon many factors, such as feed rate, millenergy, bead loading, silica particle structure, and the like. In thisembodiment, underflow stream (coarse) is recycled back to the mill whilethe overflow (fines) is collected for use or further processing.

In another embodiment, the silica particles are “double cut”, that isthe undesirable fraction of larger (coarse) particles as well as thefraction of undesirable smaller (fines) particles are removed or “cut”from the desirable sized particles. In this case, the underflow (coarse)particles are fed back into the bead mill for further particle sizereduction and the overflow (fines) particles are collected for furtherclassifying. Recycling of the underflow stream from thecentrifuge/decanter to the mill continues until the desired particlesized materials is obtained. Finally, when the particle distributionreaches the desired particle size, the overflow (fines) stream isdiscarded and the underflow (coarse) particles are collected. Thecentrifuge/decanter conditions such as feed rate, concentration, andcentrifuge speed can be adjusted to obtain the desired sized particlesin the minimum amount of time. Feed rate is generally about 0.5 LPM toabout 10 LPM. Concentration of the feed is generally about 6 wt. % toabout 50 wt. %, preferably about 10 wt. % to about 40 wt. %. Centrifugespeed is set dependent on the type of equipment used, the desiredparticle size, particle size distribution and particle size range.

The stream from either the underflow or the overflow can be collectedfor use as such or used as a feed for further optional processing. Theresultant slurry was about 6 wt % to about 50 wt % solids, preferablyabout 30 wt. % to about 50 wt. % solids.

The classifier may be configured for any range of particle sizes.Preferably, for particles to be utilized as dental abrasives, thisconfiguration permits collection of particles exhibits ranges inparticle size from 5 to 15 microns, preferably from 6 to 10, and mostpreferably from 7 to 9. For cosmetic, and other types of end-useswherein the particle sizes should be much smaller, the classifier shouldbe configured for collection of particles exhibits particle size rangesfrom 0.1 to 10 microns, preferably from 0.3 to 5, and most preferablyfrom 0.5 to 2.5.

Additionally, the classified silica slurry of step “c” (in either thefirst or second method outlined above) can be used as such, mixed withother ingredients such as a humectant, (e.g. glycerin or sorbitol),preservative, fluoride source or the like for use in a dentifrice ordewatered and washed.

In step “d” of the inventive process (optional in the first outlinedmethod, required in the second, as noted above), the classified silicaslurry of step “c” is next dewatered. In one preferred aspect, so-called“J-Vap” processing, or similar chamber filter press processing, ispreferred in the implementation of step “d” of the preferred embodimentof the invention.

Non-limiting illustrations of methods and equipment arrangements forconducting such J-Vap processing that can be adapted for use in thepractice of the filtering/dewatering step of the present invention canbe found, for example, in U.S. Pat. No. 5,558,773 and EP 0 978 304 A2,which descriptions are incorporated herein by reference. Other examplesof J-Vap processing equipment include commercially available equipmentfor this purpose, such as that illustrated in the working examplesbelow. The J-Vap processing equipment employed must permit reliable andaccurate control over the level of water removal to meet the criterionset forth herein for that parameter.

The J-Vap processing arrangement generally includes a series ofreduction chambers in which the washing and dewatering of the silicaslurry is conducted. The reduction chambers are tightly clamped togetherin the filter press module. An energy conversion module also is includedthat supplies heated water for the pressurization of the reductionchambers and also includes a vacuum system used during dewateringperformed after an initial pressure filtering stage.

In one exemplary suitable arrangement, the chamber filter press of theJ-Vap processing system is selected as including a plurality ofalternating diaphragm squeeze plates and filter plates covered byrespective liquid-permeable filter membranes, in which the squeezeplates and filter plates define abrasive suspension introduction andflow passages therebetween, wherein the squeeze plates include adiaphragm that is expandable toward an adjoining filter plate effectiveto increase solid/liquid separation in the abrasive suspension in whichliquid is transmitted through the adjoining liquid-permeable membrane,and the filter plates including respective interior filtrate drainagechambers for drainage of liquid filtered from the abrasive suspension.

During an initial filtering stage performed on abrasive slurry suppliedfrom the classifier, the slurry wet cake is washed with water, then airblow down commences to remove surface water from the cake. The initialor “core” blow down may be performed in a single step or in a pulsedmanner, wherein air is blown for a set time, stopped and repeated for aset number of times. Thereafter, the diaphragm is expanded byintroduction of heated fluid effective to expand the diaphragm and heatthe abrasive suspension effective to promote water removal from thefilter material. That is, slurry from the classifier is pumped into thereduction chambers where initial filtration occurs and the free liquidis drained away. After the initial filtration stage, vacuum-promoteddewatering is performed. For example, the reduction chambers arepressurized with heated water, and a vacuum is introduced. For example,in a second stage of the dewatering process as performed in the J-Vapprocessing system, the drainage chambers are connected to a vacuumsource effective to remove vaporized portions of the abrasivesuspension.

The dewatering time is set to achieve the desired water reduction. Afterthe dewatering stages are completed, the reduction chambers areseparated from one another and the dewatered filter cake material isdischarged and proceeds to the de-agglomeration step.

Illustrative, non-limiting conditions for conducting such J-Vapdewatering, when used to perform the dewatering step “d” according tothe preferred embodiment of the invention, include the following generalconditions:

De-water time: 0 to 6 hours;

Feed Pressure: 20-80 psi (138-552 kPa);

Feed temp: 21-85° C.;

Hot water temp: 49-85° C. (120-185° F.);

Blow down air pressure: 20-80 psi (138-552 kPa);

System Vacuum: 20-29 in. Hg (68-98 kPa);

Squeeze air pressure: 20-100 psi (138-552 kPa); and

Solids content out of J-Vap: 40-95%.

The reaction mass is filtered and washed with water to reduce the Na₂SO₄level to less than 5%, and preferably less than 2%, by weight (e.g., 0.5to 1.5%). The resulting dewatered mass generally comprises about 70 toabout 95 weight percent of silica particles, and from about 5 to about30 weight percent water (preferably 5 to 10 wt. % water). The pH of thewashed filter cake can be adjusted, if necessary.

Alternately, the classified silica of step “c” can be washed anddewatered utilizing a vacuum or pressure filter.

The dewatered silica particles resulting from step “d” generally haveagglomerated due to the pressure applied during the dewatering process.This is typically a weak particle agglomeration such that the particlesmay be separated when incorporated into a formulation, such as adentifrice or cosmetic formulation. De-agglomeration may be achieved inoptional step “e” by exposing the particles to gentle dry mixing such asairveying the particles, feeding the particles into a mill configuredwith no hammers or whizzards, high shear mixing, or the like.

The finished abrasive at step “e” (in either first or second method),having water content reduced to about less than 10% can be stored untilneeded for later usage, such in the preparation of dentifrices or othercosmetic, personal care or coating compositions.

An important aspect of this invention is that the milled, classifiedparticles (preferably abrasive silica particles) provided at step “e”can be continuously maintained at a total liquid content of at least 5wt %, up until an additional step of incorporating said abrasiveparticles into a dentifrice composition or other cosmetic, personal careor coating composition without the need to dry the silica or perform drymilling. No drying or dry milling of the precipitated silica need occurfrom the time the silica is synthesized up until its incorporation intoan oral cleaning composition. While not desiring to be bound to anyparticular theory at this time, it is postulated that drying and drymilling processes impact the surface and chemical properties of thesilica particles in unpredictable or even adverse manners, e.g.discoloration from dry milling. The present invention avoids theseimpacts of drying and dry milling.

The preferred silica particles provided in the above-illustratedabrasive compositions are preferably characterized as synthetic hydratedamorphous silicas, known as silicon dioxides or SiO₂. These precipitatedsilicas can be characterized as very low to high structure syntheticsilicas.

In addition to the above-described step “a” methodology of precipitatingthe raw synthetic amorphous silicas in the reactor, the preparation ofthe raw silica is not necessarily limited thereto and it also can begenerally accomplished in accordance with the methodologies described,for example, in prior U.S. Pat. Nos. 3,893,840, 3,988,162, 4,067,746,4,340,583, 5,225,177, 5,891,421, and 6,419,174 all of which areincorporated herein by reference, as long as such methods areappropriately modified to append the post-processing treatment(s) usedin at least steps “b” and “c” of the preferred inventive method, asdiscussed above. As will be appreciated by one skilled in the art,reaction parameters which affect the characteristics of the resultantprecipitated silica include: the rate and timing at which the variousreactants are added; the levels of concentration of the variousreactants; the reaction pH; the reaction temperature; and/or the rate atwhich any electrolytes are added.

Although silicas have been illustrated herein as the abrasive polishingagent component provided in the abrasive compositions being produced bythis invention, it will be understood that the principles of the presentinvention are also considered applicable to suspensions or slurries ofother water-insoluble abrasive particles that can be synthesized in areactor without the need for any intervening drying or dry millingsteps. Other such water-insoluble particles include, for example, silicagels, dicalcium phosphate or its dihydrate forms, calcium pyrophosphateand precipitated calcium carbonate (PCC).

Examples of use of these optional dentifrice ingredients are describedherein and/or, for example, in Reissue 29,634, and U.S. Pat. Nos.5,676,932, 6,074,629, and 5,658,553, and the patents cited therein, allbeing incorporated herein by reference. These optional ingredients, ifused, can be used at levels that are customarily seen in dentifriceformulations.

As noted above, the abrasive particles produced by this inventive method(in particular, though not required, said amorphous precipitated silicaabrasives) may then be incorporated into a dentifrice composition, e.g.,toothpaste.

In addition to the abrasive component, such a dentifrice may alsocontain several other ingredients commonly used in dentifrice makingsuch as humectants, thickening agents, (also sometimes known as binders,gums, or stabilizing agents), antibacterial agents, fluorides,sweeteners, and co-surfactants.

Humectants serve to add body or “mouth texture” to a dentifrice as wellas preventing the dentifrice from drying out. Suitable humectantsinclude polyethylene glycol (at a variety of different molecularweights), propylene glycol, glycerin (glycerol), erythritol, xylitol,sorbitol, mannitol, lactitol, and hydrogenated starch hydrolyzates, aswell as mixtures of these compounds.

Thickening agents are useful in the dentifrice compositions of thepresent invention to provide a gelatinous structure that stabilizes thetoothpaste against phase separation. Suitable thickening agents includesilica thickener, starch, glycerite of starch, gum karaya (sterculiagum), gum tragacanth, gum arabic, gum ghatti, gum acacia, xanthan gum,guar gum, veegum, carrageenan, sodium alginate, agar-agar, pectin,gelatin, cellulose, cellulose gum, carboxymethyl cellulose, hydroxyethylcellulose, hydroxypropyl cellulose, hydroxymethyl, hydroxymethylcarboxypropyl cellulose, methyl cellulose, ethyl cellulose, sulfatedcellulose, as well as mixtures of these compounds. Typical levels ofbinders are from about 0 wt % to about 15 wt % of a toothpastecomposition.

Antibacterial agents may be included to reduce the presence ofmicroorganisms to below known harmful levels. Suitable antibacterialagents include tetrasodium pyrophosphate, benzoic acid, sodium benzoate,potassium benzoate boric acid phenolic compounds such as betanapthol,chlorothymol, thymol, anethole, eucalyptol, carvacrol, menthol, phenol,amylphenol, hexylphenol, heptylphenol, octylphenol, hexylresorcinol,laurylpyridinium chloride, myristylpyridinium chloride, cetylpyridiniumfluoride, cetylpyridinium chloride, cetylpyridinium bromide. If present,the level of antibacterial agent is preferably from about 0.1 wt % toabout 5 wt % of the toothpaste composition.

Sweeteners may be added to the toothpaste composition to impart apleasing taste to the product. Suitable sweeteners include saccharin (assodium, potassium or calcium saccharin), cyclamate (as a sodium,potassium or calcium salt), acesulfame-K, thaumatin, neohisperidindihydrochalcone, ammoniated glycyrrhizin, dextrose, levulose, sucrose,mannose, and glucose.

The toothpaste will also preferably contain fluoride salts to preventthe development and progression of dental caries. Suitable fluoridesalts include sodium fluoride, potassium fluoride, zinc fluoride,stannous fluoride, zinc ammonium fluoride, sodium monofluorophosphate,potassium monofluorophosphate, laurylamine hydrofluoride,diethylaminoethyloctoylamide hydrofluoride, didecyldimethylammoniumfluoride, cetylpyridinium fluoride, dilaurylmorpholinium fluoride,sarcosine stannous fluoride, glycine potassium fluoride, glycinehydrofluoride, and sodium monofluorophosphate. Typical levels offluoride salts are from about 0.1 wt % to about 5 wt %.

Surfactants may also be included as additional cleansing and foamingagents, and may be selected from anionic surfactants, zwitterionicsurfactants, nonionic surfactants, amphoteric surfactants, and cationicsurfactants. Anionic surfactants are preferred, such as metal sulfatesalts, such as sodium lauryl sulfate.

The dentifrices disclosed herein may also contain a variety ofadditional ingredients such as desensitizing agents, healing agents,other caries preventative agents, chelating/sequestering agents,vitamins, amino acids, proteins, other anti-plaque/anti-calculus agents,opacifiers, antibiotics, anti-enzymes, enzymes, pH control agents,oxidizing agents, antioxidants, whitening agents, colorants, flavorants,and preservatives.

Finally, water provides the balance of the composition in addition tothe additives mentioned. The water is preferably deionized and free ofimpurities. The dentifrice will comprise from about 10 wt % to about 40wt % of water, preferably from 20 to 35 wt %.

Furthermore, smaller particle size particles may either be produced viathe second outlined method or as byproducts of the first outlinedmethod. Such smaller particles, preferably, again, silica particles, maybe incorporated within a variety of compositions and formulations, suchas within cosmetics and like end-uses, wherein such small particles maybe present as pigments, detackifiers, carriers, and the like.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The abrasive compositions made by the method of the present inventionare storable, ready-to-use abrasive particles and slurries that can bereadily formulated on demand with other ingredients to prepare oralcleaning compositions having a high cleaning efficacy without causingundue abrasion on tooth tissues. The essential as well as optional stepsof the inventive method are described in more detail below.

Particulate Composition Production

The following examples are presented to illustrate the invention, butthe invention is not to be considered as limited thereto. In thefollowing examples, parts are by weight unless indicated otherwise.

The silica properties described herein were measured as follows. Medianparticle size was determined using a Model LA-910 laser light scatteringinstrument available from Horiba Instruments, Boothwyn, Pa. A laser beamis projected through a transparent cell which contains a stream ofmoving particles suspended in a liquid. Light rays which strike theparticles are scattered through angles which are inversely proportionalto their sizes. The photodetector array measures the quantity of lightat several predetermined angles. Electrical signals proportional to themeasured light flux values are then processed by a microcomputer systemto form a multi-channel histogram of the particle size distribution.Samples are subjected to 2 minutes ultrasonics on the Horiba instrumentprior to analysis.

The size distribution of silica particles in a given composition isplotted as cumulative volume percent as a function of particle size.Cumulative volume percent is the percent, by volume, of a distributionhaving a particle size of less than or equal to a given value and whereparticle size is the diameter of an equivalent spherical particle. Themean particle size in a distribution is the size in microns of thesilica particles at the 50% point for that distribution. The width ofthe particle size distribution of a given composition can becharacterized using a span ratio. As used herein, the span ratio isdefined as the cumulative diameter of the particles in the tenth volumepercentile minus the cumulative volume at the ninetieth percentiledivided by the diameter of the particles in the fiftieth volumepercentile, i.e. (D10-D90)/D50.

Oil absorption, using linseed oil, was determined by the rubout method.This method is based on a principle of mixing oil with silica by rubbingwith a spatula on a smooth surface until a stiff putty-like paste isformed. By measuring the quantity of oil required to have a pastemixture which will curl when spread out, one can calculate the oilabsorption value of the silica, which is the value which represents thevolume of oil required per unit weight of silica to saturate the silicasorptive capacity. For purposes of the oil absorption measurement, thesilica sample tested was obtained directly from the silica product ofthe J-Vap procedure and dried at 105° C. for about 12 hours beforetesting. Calculation of the oil absorption value was done as follows:$\begin{matrix}{{{Oil}{\quad\quad}{absorption}} = {\frac{{ml}\quad{oil}{\quad\quad}{absorbed}}{{{weight}\quad{of}\quad{silica}},{{in}\quad{grams}}} \times 100}} \\{= {{ml}\quad{oil}\text{/}100\quad{gram}{\quad\quad}{silica}}}\end{matrix}$

A 5% pH was determined by weighing 5.0 grams silica into a 250-mLbeaker, adding 95 ml of deionized or distilled water, mixing for 7minutes on a magnetic stir plate, and measuring the pH with a pH meterwhich has been standardized with two buffer solutions bracketing theexpected pH range.

Sodium sulfate content was determined by comparison of the sampleconductivity with a standard curve generated from known sodiumsulfate/silica composition slurries. Into a one-quart mixer cup of aHamilton Beach Mixer, Model Number 30, 38 g of silica wet cake samplewas weighed, 140 ml of deionized water was added and the slurry wasmixed for 5 to 7 minutes. The slurry was transferred to a 250-mlgraduated cylinder and made up to volume to 250 ml with deionized water,using the water to rinse out the mixer cup. The sample was mixed byinverting the graduated cylinder several times. Conductivity of theslurry was determined using a conductivity meter, such as a Cole PalmerCON 500 Model #19950-00.

To measure the % 325 sieve residue, 50 g silica was weighed into a1-liter beaker containing 500-600 ml water. The silica was allowed tosettle into the water, then mixed well until all the material wasdispersed. The water pressure through the spray nozzle (Fulljet 9.5, ⅜G, 316 stainless steel, Spraying Systems Co.) was adjusted to 20-25 psi.The sieve screen cloth (325 mesh screen, 8″ diameter) was held 4-6inches below the nozzle and, while spraying, the beaker contents wasgradually poured onto the 325 mesh screen. The remaining material wasrinsed from the walls of the beaker and poured onto the screen. Thescreen was washed for 2 minutes, moving the spray from side to side inthe screen using a sweeping motion. After spraying for 2 minutes (allparticles smaller than the screen opening should have passed through thescreen), the residue retained on the screen was washed to one side, andthen transfer into a pre-weighed aluminum weighing dish by washing withwater from a squirt bottle. A minimum amount of water needed to be sureall the residue was transferred into the weighing dish was used. Thedish was allowed to stand 2-3 minutes (residue settles), then the clearwater was decanted off the top. The dish was placed in an oven(“Easy-Bake” infrared oven or 105° C. oven) and dried until the residuesample was dried to a constant weight. $\begin{matrix}{{\%\quad 325\quad{residue}} = {\frac{{{weight}{\quad\quad}{of}{\quad\quad}{residue}},g}{{{sample}\quad{weight}},g} \times 100}} \\{= {{\%\quad{particles}} > {45\quad\mu\quad m}}}\end{matrix}$

To determine loose bulk density, 100 g of silica was gently poured intoa 100-mm polypropylene powder funnel clamped with the aperture 1.5inches above the top of a 250-ml polymethylpentene graduated cylinder,which had been cut off at the 100-ml marking and pre-weighed. The flowof silica is stopped when the cylinder was overflowing. The powder inthe graduated cylinder was immediately leveled by scraping across thetop of the cylinder with a spatula. This step should be done as quicklyas possible to prevent settling of the powder bed which would giveartificially high values. Excess powder was brushed from the base of thegraduated cylinder and the filled cylinder was weighed to 0.01 gaccuracy. Volume change in the cylinder due to handling is ignored.Loose Bulk Density(g/ml)=(total wt.−initial wt.)/100

Pack or tapped density was determined by weighing 20.0 grams of productinto a 250-mL plastic graduated cylinder with a flat bottom. Thecylinder was closed with a rubber stopper and placed on a tap densitymachine and run for 15 minutes. The tap density machine is aconventional motor-gear reducer drive operating a cam at 60 rpm. The camis cut or designed to raise and drop the cylinder a distance of 2.25inch (5.715 cm) every second. The tapped density was calculated as thevolume occupied by a known weight of product and expressed in Mercurypore volume was determined using an Autopore II 9220 Porosimeter(Micromeritics Corporation). This instrument measures the void volumeand pore size distribution of various materials. Mercury is forced intothe voids as a function of pressure and the volume of mercury intrudedper gram of sample is calculated at each pressure setting. Total porevolume expressed herein represents the cumulative volume of mercuryintruded at pressures from vacuum to 60,000 psi.

To measure brightness, fine powder materials pressed into a smoothsurfaced pellets were evaluated using a Technidyne Brightmeter S-5/BCavailable from Technidyne Corporation, New Albany, Ind. This instrumenthas a dual beam optical system where the sample is illuminated at anangle of 45°, and the reflected light viewed at 0°. It conforms to TAPPItest methods T452 and T646, and ASTM Standard D985. Powdered materialsare pressed to about a 1 cm thick pellet with enough pressure to give apellet surface that is smooth and flat and without loose particles orgloss.

The Brass Einlehner (BE) Abrasion value was measured through the use ofan Einlehner AT-1000 Abrader. In this test, a Fourdrinier brass wirescreen is weighed and exposed to the action of a 10% aqueous silicasuspension for a fixed number of revolutions, and the amount of abrasionwas then determined as milligrams brass lost from the Fourdrinier wirescreen per 100,000 revolutions. Disposable supplies required for thistest (brass screens, wear plates and PVC tubing) are available fromDuncan Associates, Rutland, Vt. and sold as an “Einlehner Test Kit”.Specifically, brass screens (Phosphos Bronze P.M.) were prepared bywashing in hot, soapy water (0.5% Alconox) in an ultrasonic bath for 5minutes, then rinsing in tap water and rinsing again in a beakercontaining 150 ml water set in an ultrasonic bath. The screen was rinsedagain in tap water, dried for 20 minutes in an oven set at 105° C.,cooled in a desiccator and weighed. Screens are handled with tweezers toprevent skin oils from contaminating the screens. The Einlehner testcylinder was assembled with a wear plate and weighed screen (red lineside down—not abraded side.) and clamped in place. The wear plate wasused for about 25 tests or until worn badly; the weighed screen was usedonly once.

A 10% silica slurry, prepared by mixing 100 g silica with 900 gdeionized water, (or in the case of silica slurry, 227 g of silicaslurry at 45% solids was mixed with 773 g water) was poured into theEinlehner test cylinder. Einlehner PVC tubing was placed onto theagitating shaft. The PVC tubing has 5 numbered positions. For each test,the position of the PVC tubing was incremented until it has been usedfive times, then discarded. The Einlehner abrasion instrument wasre-assembled and the instrument set to run for 87,000 revolutions. Eachtest takes about 49 minutes. After the cycle was completed, the screenwas removed, rinsed in tap water, placed in a beaker containing waterand set in an ultrasonic bath for 2 minutes, rinsed with deionized waterand dried for 20 minutes in an oven set at 105° C. The dried screen wascooled in a desiccator and re-weighed. Two tests were run for eachsample and the results were averaged and expressed in mg lost per100,000 revolutions. The result for a 10% silica slurry, measured inunits of mg lost per 100,000 revolutions, can be characterized as the10% brass Einlehner (BE) abrasion value.

Silica structure as used herein is described in the article “CosmeticProperties and Structure of Fine-particle Synthetic PrecipitatedSilicas”, S. K. Wason, in the Journal of Soc. Cosmet. Chem., Vol. 29,(1978), pp. 497-521, which is incorporated herein by reference. Suchinventive compositions include silica particles that exhibit a linseedoil absorption value of from about 50 ml/100 g to about 90 ml/100 g.

EXAMPLE 1

A batch of amorphous precipitated silica was prepared in a reactor asfollows, which thereafter was subjected to certain dewatering, wetmilling and particle classification described below, without any dryingor dry milling occurring, to observe the effects of the post-processingprocedures that were applied.

The batch was prepared by adding 502 gallons (1900 L) of sodium silicate(13.0%, 2.50 mole ratio of SiO₂:Na₂O,) to a reactor and heated it to 85°C. Then sodium silicate (13.0%, 2.50 mole ratio of SiO₂:Na₂O, preheatedto 85° C.) and sulfuric acid (11.4%) were simultaneously added to thereactor at a rate of 102.4 gpm (387.6 L/min) and 45.0 gpm (170.3 L/min).The simultaneous addition of sodium silicate and sulfuric acid continuesfor 48 minutes, after which time the sodium silicate addition isdiscontinued. The acid flow was continued until the batch pH dropped to5.2, at which time the acid flow was stopped. The batch was thendigested at 93° C. for 10 minutes, with the pH adjusted back towards 5.2as needed throughout digestion. After digestion, the pH was manuallyadjusted to 5.1±0.1, and the batch was discharged from the reactor intoa filter feed tank.

The batch was filtered on a pressure filter press (model JVAP 470/100available from US Filter Corporation, Holland, Mich.) to 33-34% solidsand then the press cake was fed into a Premier bead mill (model HML-1.5available from Lightnin, Inc., Reading, Pa.) at a rate of 0.43 litersper minute (LPM). The Premier mill is a horizontal style media mill,having a 1.5 liter grinding chamber which was loaded with 1.06 liters of0.8-1.0 mm sized zirconia media beads having a specific gravity of 3.7.The silica was passed through the mill three times with an aliquotcollected after each pass to measure particle size characteristics. Theparticle size properties of the starting feed and bead milled materialsmeasured according to the methods described above are summarized belowin Table 1. TABLE 1 Pass No. MPS, μm Span Starting Feed 1.5.34 3.10 10.87 5.77 2 0.47 3.61 3 0.46 2.87it is seen in Table 1 that bead milling was effective to reduce theparticle size of Example 1 silica from about 15 μm to about 0.5 μm.

Next, the milled press cake from the third pass was split into 2portions which were separately fed into a Sarples BM-PF743/54893C3decanter/centrifuge under the conditions summarized in Table 2 below.

In the first trial, five gallons of the bead milled silica preparedabove was diluted with 15 liters of water in the feed tank (20% solids)and mixed. Four separate trials (Trial 1A-1D) were conducted under thecentrifuge/decanter conditions given in Table 2. The centrifuge/decanterwas configured to recirculate both the overflow and underflow streams.Recirculation continued for the duration of the Trials 1A through 1D.

In the second trial, five gallons of the bead milled silica preparedabove was diluted with 50 liters of water to 9.8% solids and mixed. Thecentrifuge/decanter was configured to recirculate both the overflow andunderflow streams for the duration of separate Trials 2A through 2D. Atthe end of the Trial 2D, recirculation was turned off so that theoverflow stream could be collected for Trail 3.

In Trial 3, ten gallons of the overflow material from Trial 2D (9.8%solids) was fed into the centrifuge/decanter configured so that theoverflow was recirculated and the underflow was discarded. Thecentrifuge was run at 60 Hz with samples collected for particle sizeanalysis every 0.5 hour from 2 to 4 hours. TABLE 2 Centrifuge Feed RateTrial No. Time, Hr % Solids Hz LPM 1A 0.5 20 60 0.5 1B 0.5 20 30 1 1C0.5 20 60 1 1D 0.5 20 30 0.5 2A 0.5 9.8 60 0.5 2B 0.5 9.8 30 1 2C 0.59.8 60 1 2D 0.5 9.8 30 0.5 3A 0 9.8 60 0.5 3B 2 9.8 60 0.5 3C 2.5 9.8 600.5 3D 3.0 9.8 60 0.5 3E 3.5 9.8 60 0.5 3F 4 9.8 60 0.5

Particle size characteristics of materials from Trials 1, 2 and 3measured according to the methods described above are summarized inTable 3. TABLE 3 Trial No. MPS, μm Span 1A 0.28 0.72 1B 0.41 2.33 1C0.28 0.75 1D 0.32 0.90 2A 0.28 0.83 2B 0.40 1.90 2C 0.33 0.96 2D 0.371.40 3A 0.36 1.45 3B 0.19 0.58 3C 0.17 0.59 3D 0.17 0.58 3E 0.16 0.59 3F0.15 0.59

It is seen that centrifuging this bead milled silica was effective toisolate fractions of smaller particle size materials, even as small as0.15 μm with a very narrow particle size distribution as indicated by asmall span value.

EXAMPLE 2

A portion of the silica prepared in Example 1 was collected after thereactor, filtered and washed to remove sulfate, then the filter cake wasfed at a rate of 0.8 LPM into a 1.5 liter horizontal style media mill(model HML 1.5 Premiere Mill manufactured by Lightnin, Inc., Reading,Pa.) loaded with 1080 ml of 1.6 mm zirconia beads having a specificgravity of 3.7 and set to a speed of 2500 FPM (feet per minute) toreduce the particle size. The silica feed (Example 2A) to the mill had aMPS of 10.8 μm and a distribution span of 4.8. After milling the silicadenoted as Example 2B had a MPS of 5.79 μm, a span of 2.31 and was at34.9% solids. Next, the milled slurry was fed into a 6″ solid bowlcontinuous flow 6,000 rpm Bird centrifuge (BIRD Machine Company Inc.,South Walpole, Mass.) in order to collect a fraction of particles with aMPS of about 6 μm. The silica slurry was fed into the centrifuge at arate of 8.9 LPM with the centrifuge set to 40 HZ. The overflow wascollected and fed beck into the centrifuge (60 HZ) at a rate of 3 LPM.From this classification, the underflow was collected as Example 2C andfed to a J-Vap dewatering system (model JVAP 470/100 available from USFilter Corporation, Holland, Mich.) for dewatering the silica slurry.The slurry feed was at 45% solids before dewatering and at 90% solidsafterwards. The dewatered silica from the J-Vap dewatering system isdenoted as Example 2D. The J-Vap dewatering conditions are summarized inTable 4. TABLE 4 J-Vap Dewatering Conditions Fill Pressure, psig 80Slurry Temp., ° C. 67 Volume, L 7.5 Time, min 2 1rst Blow Down (BD) BDP, psig 20 BD Time, min 2 Core Blow Down (BD) P, psig 80 Volume, L 1 BDTime, sec 15 Number BD 3 2nd Blow Down (BD) P, psig 60 Volume, L 1 Time,min 10 Squeeze P, psig 95 Water Temp, ° C. 85 Vacuum, inch Hg 28 DryTime, hr 5 Cake Solids, % 90.03 Cake Weight, kg 7.5

Finally, the material was discharged from the J-Vap dewatering systemand then fed into a Raymond mill configured with no hammers or whizzardsto gently de-agglomerate the particles. The de-agglomerated particlesare designated as Example 2E.

Another portion of the silica of Example 1 was collected after thereactor then filtered, washed, spray dried and milled (2 Control) to beused as a dry silica control representing conventionally preparedprecipitated silica.

Properties of the silicas from all stages of Example 2 and theconventionally prepared silica “Control 2” were measured according tothe procedures described above and are summarized below in Table 5.TABLE 5 Example 2 2A 2B 2C 2D 2E Control % Solids 34.9 34.9 45.0 91.595.1 95.8 MPS, μm 10.8 5.8 6.0 7.2 6.3 8.3 Span 3.69 2.31 1.94 2.07 1.691.95 % 325 Mesh Residue — — 0.04 2.37 0.02 0.00 5% pH — — 7.3 7.9 7.87.1 Pore Volume, ml/g — — 1.14 1.25 1.57 1.76 Oil Absorption, — — — 7578 91 ml/100 g Pack Density, g/ml — — — 0.64 0.50 0.42 Pour Density,g/ml — — — 0.45 0.24 0.21 BE, mg loss/100,000 — — 3.0705 — 6.9345 5.0485rev Brightness — — 99.6 99.5 97.8 96.0

The data summarized in Table 5 above shows that the inventive processwas effective to provide a silica product with sequentially reducedsilica median particle size and a narrow particle size distribution(span) by bead milling the silica slurry, classifying the resultingslurry and then dewatering and gently de-agglomerating the resultingsilica product without any intervening expensive drying (spray or nozzledrying) step. It is surprising to note that Example 2E produced by theinventive process has a lower oil absorption value, higher BrassEinlehner value and is more dense than conventionally produced silica(Control 2) indicating the inventive process produces a lower structure,more abrasive silica. Also surprisingly, the inventive silica Examples2C-2E have a higher brightness value than the conventionally producedControl 2 silica showing that the inventive silicas, while even lowerstructure (harder) than Control 2 silica, can be wet bead milled,classified, dewatered and de-agglomerated to essentially the same sizeas the conventional silica without intensive dry milling, resulting in aloss in brightness. The silica product can be used in the slurry formfrom step 2C or used as a dewatered and de-agglomerated silicacontaining as little as about 5% moisture from step 2E.

EXAMPLE 3

This batch of amorphous precipitated silica was prepared by adding 474gallons (1794 L) of sodium silicate (13.30%, 2.65 mole ratio ofSiO₂:Na₂O) to a reactor and heating it to 85° C. Then, simultaneousaddition of sodium silicate (13.3%, 2.65 mole ratio of SiO₂:Na₂O,preheated to 85° C.) at a rate of 92.7 gpm (351 L/min) and sulfuric acid(11.4%) at a rate of 41.5 gpm (157 L/min) was continued for 47 minutes,after which time the sodium silicate addition was discontinued. The acidflow was continued until the batch pH dropped to 5.9±0.1, at which timethe acid flow was discontinued. The batch was then digested at 93° C.for 10 minutes, with the pH adjusted back towards 5.9 as neededthroughout digestion. After digestion, the pH was manually adjusted to5.9±0.1, and the batch was discharged from the reactor.

Example 3 silica had a 5% pH of 7.75, a sodium sulfate content of 0.9%,an oil absorption of 73 ml/100 g and a median particle size of 27 μm.

Example 3 silica prepared as described above was diluted to 40% solidswith water and split into several portions for post reaction particlesize reduction in a Denver mill (Denver Equipment, Colorado Springs,Colo.) and particle classification in a Bird solid bowl centrifugeavailable from Bird Machine Company, South Walpole, Mass.

Example 3 silica, having a median particle size of 27 μm before anymilling commenced, was the starting silica fed to the mill for Examples3A-3F. Example 3A, having a median particle size of 10.9 μm aftermilling and before any classification took place, was milled under theconditions given in the Table 6 and then was fed to the centrifuge underthe conditions given for Examples 3B and 3C. Examples 3D-3F were milledand classified under the conditions shown in Table 6. The Denver milland Bird centrifuge classification conditions used for each trial aresummarized in Table 6. TABLE 6 Mill Conditions Feed CentrifugeConditions Rate % bead Speed. Feed Rate Ex. No. LPM loading HZ LPM RPM3A 3.78 20 50 NA NA 3B 3.78 20 50 3.78 1440 3C 3.78 20 50 2.84 1500 3D1.89 20 67.5 1.89 1440 3E 1.89 20 70 1.89 1380 3F 1.89 40 70 1.89 2240

Several properties of the silicas from each mill and classificationtrial were determined according to the methods described above and aresummarized below in Table 7. TABLE 7 Oil Example absorption MillCentrifuge Centrifuge No. ml/100 g MPS, μm MPS, μm Span top size, μm 3A76 10.9 NA NA NA 3B 78 10.9 9.0 4.10 — 3C 84 10.9 4.8 3.30 — 3D 77 5.62.6 4.13 17.4 3E 71 5.0 3.8 2.62 26.1 3F 65 5.9  0.54 2.50 10.1

The milling was effective in reducing the median particle size from 27μm to about 5 μm. Classification was effective to isolate a fraction ofthe particles having a narrow particle size distribution by removinglarge particles. The feed to the centrifuge had a top size of about 450μm.

Dentifrice Formulations

Toothpaste formulations were prepared to demonstrate the ready-to-use ondemand capabilities of the inventive abrasive particulate compositions.Dentifrices were formulated with the silica made by the inventiveprocess, as described above, and other ingredients, in amounts indicatedin grams, as described in TABLE 8 below. For comparison, the Control 2dry silica prepared in Example 2 and Example 2B silica were separatelyincorporated in toothpaste formulations. Properties of these dentifriceformulations are given in TABLE 9 below.

To prepare the dentifrices, the following procedure was followed. Theglycerin, sodium carboxymethyl cellulose (Cekol 2000 from the Noviant B.V., Nijmegen, The Netherlands) and sorbitol were mixed together andstirred until the ingredients were dissolved to form a first admixture.The deionized water, sodium fluoride, tetrasodium pyrophosphate andsodium saccharin were also mixed together and stirred until theseingredients are dissolved to form a second admixture. These twoadmixtures were then combined with stirring to obtain a “pre-mix”.

The pre-mix was placed in a Ross mixer (Model 130 LDM) and abrasivesilica and titanium dioxide were mixed in without vacuum. A 30-inchvacuum was drawn and the resultant admixture was stirred forapproximately 15 minutes. Lastly, sodium lauryl sulfate and flavor wereadded and the admixture was stirred for approximately 5 minutes at areduced mixing speed. The resulting dentifrice composition is sealed intoothpaste tubes and held under appropriate conditions for latertesting. TABLE 8 Toothpaste No. 1 2 3 4 Glycerin, 99.5%, g 11.600 11.600  11.600  11.600  Sorbitol, 70.0%, g 38.457  38.457  38.457 38.457  Deionized Water, g 25.000  0.600 25.000  0.000 Cekol 2000 CMC, g1.200 1.200 1.200 1.200 Tetrasodium 0.500 0.500 0.500 0.500pyrophosphate, g Sodium Saccharin 0.300 0.300 0.300 0.300 SodiumFluoride 0.243 0.243 0.243 0.243 Abrasive Example 2E 20.0   — — —Example 2C — 44.4   — — Example 2 control — — 20.0   — Example 2B — — —45.0   TiO₂, g 0.500 0.500 0.500 0.500 Sodium Lauryl Sulfate 1.200 1.2001.200 1.200 Flavor 1.000 1.000 1.000 1.000

The toothpaste properties described herein were measured as follows,unless indicated otherwise.

The toothpaste viscosity is measured utilizing a Brookfield ViscometerModel RVT equipped with a Helipath T-F spindle and set to 5 rpm bymeasuring the viscosity of the toothpaste at 25° C. at three differentlevels as the spindle descends through the toothpaste test sample andaveraging the results. Brookfield viscosity is expressed in centipoise(cP).

The pH values of the toothpaste mixtures (25 weight %) encountered inthe present invention can be monitored by any conventional pH sensitiveelectrode.

Aesthetic properties of toothpaste (stand-up, separation) were measuredvisually. About a one inch ribbon of toothpaste was squeezed from a tubeonto a piece of ordinary white notebook paper. After waiting 3-5minutes, aesthetic property observations were recorded.

Stand-up refers to the shape of the toothpaste ribbon and relates to thepaste's ability to stay on top of a toothbrush without sinkingin-between the bristles. A scale of 1-10 is used, with a 10 stand-uprating being good and meaning the ribbon retained its shape; a rating of1 for stand-up is poor meaning the ribbons flattens out, losing itsshape.

Separation refers to the toothpaste formulation's integrity. Solid andliquid phases of the toothpaste may separate, usually due to too littlebinder or thickener. Liquid will be visible around the squeezed ribbonof paste if there is separation. Separation ratings are on a scale of1-10 with a rating of 10 meaning no separation; a rating of 1 meaningmajor phase separation; and intermediate ratings meaning that there anamount of liquid appears around ribbon.

The Radioactive Dentin Abrasion (RDA) values of the precipitated silicacompositions used in this invention are determined according to themethod set forth by Hefferen, Journal of Dental Res., July-August 1976,55 (4), pp. 563-573, and described in Wason U.S. Pat. Nos. 4,340,583,4,420,312 and 4,421,527, which publications and patents are incorporatedherein by reference.

The PCR test is described in “In Vitro Removal of stain With Dentifrice”G. K. Stookey, et al., J. Dental Res., 61, 1236-9, 1982. TABLE 9Toothpaste No. 1 2 3 4 24 hr. 190,000 210,000 190,000 220,000 Viscosity,cP 1 Wk 220,000 240,000 240,000 280,000 Viscosity, cP 3 Wk 260,000280,000 250,000 300,000 Viscosity, cP 6 Wk 290,000 300,000 280,000310,000 Viscosity, cP 25% pH 7.26 7.15 7.11 7.13 RDA 146 140 135 130 PCR83 85 79 81

Toothpaste formulated with the inventive silica slurry compositions hadgood viscosity and aesthetic properties. The ability to provide moderateRDA levels with simultaneously high PCR measurements thus shows thebenefits of such abrasive particle compositions in dentifriceformulations as well. Thus, the wet comminution with subsequentclassification of resultant particles provides heretofore unmet benefitsat least in terms of production complexity and lower overall productioncosts to provide a substantially similar product to those currentlyutilized within the dentifrice industry.

It will be understood that various changes in the details, materials,and arrangements of the parts which have been described and illustratedherein in order to explain the nature of this invention may be made bythose skilled in the art without departing from the principles and scopeof the invention as expressed in the following claims.

1. A method of providing a particulate composition, said methodcomprising the sequential steps of: a) providing a plurality ofparticles selected from the group consisting of precipitated silicaparticles, silica gel particles, precipitated calcium carbonateparticles, calcium pyrophosphate particles, dicalcium phosphate, and anymixtures thereof; b) subjecting said plurality of particles to acomminuting step in a wet environment; c) subjecting said wet-comminutedparticles of step “b” to a particle size classification step whereinparticles exhibiting a median particle size range of from about 5 toabout 30 microns are collected; d) subjecting said collected particlesfrom step “c” to a subsequent particle size classification step whereinparticles exhibiting a median particle size range of from about 5 toabout 15 microns are collected; optionally e) subjecting said collectedclassified particles from step “d” to a dewatering step wherein saiddewatered particles exhibit a moisture content of at most 60 wt %; and,optionally f) subjecting said dewatered abrasive particles to adeagglomeration step.
 2. The method of claim 1 wherein said collectedclassified particles of step “d” exhibit a particle size span of lessthan or equal to
 2. 3. The method of claim 2 wherein said collectedclassified particles from step “d” exhibit a median particle size ofabout 6 to about 10 microns and a particle size span of from about 1.25to about 2.0.
 4. The method of claim 3 wherein said collected classifiedparticles from step “d” exhibit a median particle size of about 7 toabout 9 microns, a particle size span of from about 1.25 to about 1.95.5. The method of claim 1 wherein step “e” is present.
 6. The method ofclaim 2 wherein step “e” is present.
 7. The method of claim 3 whereinstep “e” is present.
 8. The method of claim 4 wherein step “e” ispresent.
 9. The method of claim 5 wherein step “f” is present.
 10. Themethod of claim 6 wherein step “f” is present.
 11. The method of claim 7wherein step “f” is present.
 12. The method of claim 8 wherein step “f”is present.
 13. An abrasive particle composition as produced by themethod of claim
 1. 14. An abrasive particle composition as produced bythe method of claim
 2. 15. An abrasive particle composition as producedby the method of claim
 3. 16. An abrasive particle composition asproduced by the method of claim
 4. 17. An abrasive particle compositionas produced by the method of claim
 5. 18. An abrasive particlecomposition as produced by the method of claim
 9. 19. The method ofclaim 1 wherein said particles are amorphous precipitated silicaparticles and step “a” comprises the admixing of a sufficient amount ofan alkali silicate and an acidulating agent to form precipitated silicaparticles.
 20. The method of claim 2 wherein said particles areamorphous precipitated silica particles and step “a” comprises theadmixing of a sufficient amount of an alkali silicate and an acidulatingagent to form precipitated silica particles.
 21. The method of claim 3wherein said particles are amorphous precipitated silica particles andstep “a” comprises the admixing of a sufficient amount of an alkalisilicate and an acidulating agent to form precipitated silica particles.22. The method of claim 4 wherein said particles are amorphousprecipitated silica particles and step “a” comprises the admixing of asufficient amount of an alkali silicate and an acidulating agent to formprecipitated silica particles.
 23. The method of claim 5 wherein saidparticles are amorphous precipitated silica particles and step “a”comprises the admixing of a sufficient amount of an alkali silicate andan acidulating agent to form precipitated silica particles.
 24. Themethod of claim 9 wherein said particles are amorphous precipitatedsilica particles and step “a” comprises the admixing of a sufficientamount of an alkali silicate and an acidulating agent to formprecipitated silica particles.
 25. The precipitated silica particlesproduced by the method of claim
 19. 26. The precipitated silicaparticles produced by the method of claim
 20. 27. The precipitatedsilica particles produced by the method of claim
 21. 28. Theprecipitated silica particles produced by the method of claim
 22. 29.The precipitated silica particles produced by the method of claim 23.30. The precipitated silica particles produced by the method of claim24.